Introduction
Vibration Testing
It should be easy to understand the importance of design suitability where a product needs to operate in an environment where operation or transportation is required in its normal operational life. Prime examples of these products are automotive electronics, aerospace electronics, portable personal electronics such as telephones, cameras or entertainment systems.
Unfortunately it is frequently assumed that products such as computers, televisions, lab based test equipment or static electronics systems are immune from vibration related defect induction. Throughout the manufacturing process through to packaging, shipment and delivery, practically every product is moved and handled by fork lift, air, road, rail and by trolley.
- Continuous vibration such as road or air related transportation can exciteand maintain structural or component resonances resulting infatiguing of weak joints, loosening of screws and connectors and physical damage to poorly designed products.
- Transient vibration such as a pallet of goods being handled roughly or by a forklift truck being driven over a rough surface can induce failure in almost every part of the product.
How are vibration environments simulated?
An electrodynamic shaker (vibrator) is used to physically create the vibration environment. products are attached using a purpose designed fixture and a vibration environment is created and controlled by a computer based system. There are two prime types of vibration used in stress screening:
- Sine.
- Random.
Sine excitation is a traditional form of test. A simple sine wave (single frequency) is generated and applied to the shaker. The sine may be swept, in which case the exciting frequency is continuously changing across a pre defined range.
Random excitation is a more recent evolution and creates a more lifelike vibration environment by generating and controlling all frequencies simultaneously, The amplitudes of the random excitation at each frequency are varying with a gaussian random distribution.
Both random and sine are in use today, each has its own advantages. Sometimes both techniques are combined to create a "mixed mode" test in which both sine and random excitation is applied and controlled individually and simultaneously. This type of test can be effective for screening of components for use in applications such as helicopters, engines or machinery.
Well designed and well manufactured products withstand the repeated stress reversals of vibration without failure. However, flaws can become concentration areas for the stresses which tend to concentrate around the defect, with the result that failure occurs.
How can latent defects be identified before the product leaves your factory?
By performing vibration stress screening both during the design phase and in the production stage of a product, the same failure mechanisms seen by the product in real life can be forced to occur in the lab. It is important to remember that it is not necessarily the requirement for a product to survive and operate at a particular vibration level that is the problem. Weak or badly installed compo nents, or badly made solder joints will ultimately become failure points. Combine that with thermal stressing, condensation and air born contaminants and the potential for shortened life can be anticipated quite easily.
Testing in controlled conditions can enable accelerated testing by the application of vibration screening in the lab. The process is well established and as with most other technologies is being continuously refined.
Although this text primarily addresses the use of production testing in the production process, there is an underlying requirement that the product has been properly designed and is capable of surviving in use. Vibration stress screening in the production line is a tool for exposing manufacturing or process control defects once a robust and suitable design hasbeen achieved. Similar technology can be applied to testing in the design phaseand production quality testing should always be considered from the earlieststages of product design. It is too late to discover fundamental design flawsonce the product is in production.
Establishing the right vibration screen !
Anything can be reduced to failure with a vibration screen which is too severe. The emphasis is on effective screening, developing a vibration profile which exposes early life failures without damage to the product. A fully controlled vibration test using an electrodynamic shaker is the most effective tool available today. Naturally, consideration must be made to its size, structure operating environment and functionality. When anything is tested on a shaker the environment applied must be enough to weed out early life failures yet not so high as to shorten the product life. Testing with the product powered up can increase the effectiveness of the screen and sometimes vibration screening combined with thermal testing can be an optimum method. If you think that vibration screening might help to improve your products you need to know which questions to ask and consider before answers from anyone can be trusted.
Where do I go to obtain impartial advice and guidance about setting up a vibration testing regeime ?
Talk to us at the SEE first, we run regular courses on all aspects of environmental engineering.